How Seagate's Terabit-Per-Square-Inch Hard Drive Works

Heat-assisted magnetic recording promises 60-terabyte hard disks.

Magnetic hard disks will soon be able to store one terabit (a trillion bits) per square inch. Seagate has demonstrated that landmark storage density using a new magnetic recording method that can cram 10 terabits, and perhaps even more, onto every inch of a standard 3.5-inch disk. Disks made with current technology can hold about 3 terabytes.

The technology, called heat-assisted magnetic recording, involves heating the magnetic regions on a disk that hold individual data bits, allowing those regions to be made tinier. Seagate says the method promises to keep increasing storage density, and it could lead to 60-terabyte hard drives.

“One of the most exciting things about heat-assisted magnetic recording is that it’s in its infancy,” says Ed Gage, principal technologist of heads and media R&D at Seagate. The company is targeting 2015 for its first commercial product featuring the technology.

Today’s hard disks are made of magnetic cobalt-platinum alloys. Each bit is stored on a tiny area with a magnetic field pointing in one of two opposite directions, denoting a binary digit 1 or 0. The smaller these magnetized areas are, the higher the density of the disk. When the areas get down to 25 nanometers to a square side (corresponding to 1 terabit per square inch), they become unstable, meaning that a small amount of heat can make them flip their magnetic field direction.

More-stable magnetic materials, such as iron-platinum alloys, are available, says Mark Kryder, an electrical and computer engineering professor at Carnegie Mellon University and previous CTO of Seagate. However, to write on them requires magnetic fields much larger than those conventional recording heads can produce. If, however, you heat the material, smaller magnetic fields will work. So heat-assisted recording involves heating iron-platinum disks with a short laser pulse when the head applies a magnetic field to write data.

That is exactly what Seagate has done. Three years ago they demonstrated 250 gigabits per square inch density using the technology. Since then, Gage says, they’ve made substantial improvements in two areas: the recording head and the iron-platinum medium.

The biggest issue with the new head is that it needs to focus light onto 25-nanometer-wide spots, which is tough with conventional lens-based optics. So Seagate uses a parabolic mirror that focuses light down to a quarter of its wavelength, making 100-nanometer spots. To tighten that even more, Seagate researchers use a tiny gold antenna that collects light and reemits it at a 30-nanometer spot. “It’s a gold piece that has to be appropriately shaped,” Gage says. “We’ve tried a number of different antenna shapes.”

The iron-platinum medium poses its own difficulties. “You need a smooth platter, a very good granular microstructure,” Gage says. “You have to be able to grow the right crystalline structure.” Plus, he says, heat spreads in the magnetic material. “You have to build layers in there to control the way the heat flows laterally and vertically.”

Right now, Seagate uses an external laser to shine light on the parabolic mirror. But Gage says they have already put a laser inside a recording head.

Nonetheless, Gage says, much more work is needed before Seagate has a commercial product: “Putting together the head, magnetic media, [electronic control circuits], and firmware and getting them into a hard drive is significant work.”